1. Field of the Invention
This invention relates to an image forming apparatus adapted to form an image on recording sheets sent to a predetermined position in order from a sheet tray, and more particularly to an image forming apparatus capable of forming an image in an accurate position on each of the recording sheet even when a tab and the like is provided at a front end of the recording sheet sent to the mentioned position.
2. Background Art
In an image forming apparatus adapted to form an image on a recording sheet, especially, in an image forming apparatus adapted to form a visible image on a recording sheet by selectively depositing a powdered toner on a latent image formed owing to an electrostatic potential difference, a means for regulating the recording sheet sending time so as to form an image in an accurate position on the recording sheet is employed. In an electrophotographic image forming apparatus adapted to form an image by using a toner image, and, in an electrostatic recording type image forming apparatus, a toner image is formed on an image carrier, and then sent forward by the movement of a circumferential surface of the image carrier. This toner image is then transferred onto a recording sheet directly or via an intermediate transfer member. During this time, the time at which the recording sheet is sent forward is regulated with respect to the movement of the member on which the toner image is supported, and the recording sheet is brought into contact with an accurate portion of the member on which the toner image is supported. This time regulation is conducted generally by using resist rolls which will be described below.
As shown in FIG. 4, on the upstream side of a transfer unit 101 in which the toner image is transferred onto the recording sheet, two resist rolls 102 arranged in parallel with each other and engaged each other under pressure are provided. When a recording sheet P is sent from a sheet tray 103, a front end of the recording sheet is made to impinge upon pressure engaged portions of the two resist rolls 102 stopped in advance. During this time, after the front end of the recording sheet contacts the resist rolls 102, further upstream side sending rolls 104 feed out the recording sheet P by a predetermined length, and stop with a rear portion of the recording sheet P gripped. As a result, flexure occurs between the sending roll 104 and resist rolls 102 as shown in FIG. 4, and the front end of the recording sheet is pressed against the resist rolls 102 by an elasticity restoration force of the recording sheet itself. Even though the recording sheet is sent in an inclined state during this time, the recording sheet P has a marginal length between the sending rolls 104 and resist rolls 102, so that the front end of the recording sheet P is pressed against the resist rolls 102 with the recording sheet extending at right angles to the axes of the resist rolls.
The driving of the resist rolls 102 is then started in accordance with the time at which the toner image is sent thereto, to send the recording sheet P forward. During this time, the front end of the recording sheet is held between the two resist rolls 102 at right angles to the axes thereof, i.e., the recording sheet is fed forward with a diagonal posture thereof corrected. The recording sheet is brought into contact with an accurate portion of the toner image being sent thereto, and the toner image is transferred onto the recording sheet.
However, the recording sheets used have been diversified in recent years, and a recording sheet the front end of which does not extend straight at right angles to the direction in which the front end is sent is used in some cases. In a case where such a recording sheet is used, hindrance to the sending of the recording sheet occurs. For example, when a so-called tab sheet on which a tab for putting an index thereon is provided in a projecting state is made to impinge upon the resist rolls with the tab projecting from a front end of the tab sheet, the tab sheet which has theretofore been sent forward accurately starts being sent diagonally due to the projecting tab. Moreover, the time during which the recording sheet is sent forward is delayed by a period of time corresponding to the length of the tab, so that an image cannot be transferred onto an accurate portion of the recording sheet.
In order to deal with such problems, when the forming of an image is done by not disposing the edge, on which the tab is provided, of the tab sheet on the forward side, inconveniences do not occur. However, when images are formed on both surfaces of a tab sheet, the direction in which the tab sheet is sent is reversed. Therefore, in order to transfer an image onto the rear surface of the tab sheet, the tab sheet is sent with the edge thereof on which the tab is provided directed forward, though the tab was positioned at the rear end of the tab sheet during the formation of an image on an outer surface of the tab sheet. When a tab sheet is sent so that the tab is positioned on the lateral side thereof, inconveniences occur in a certain case where after-treatments, such as hole making operations and sheet binding operations are carried out continuously.
Other means for solving the above problems are disclosed in JP-A-2003-226448 and JP-A-2003-122223. In the image forming apparatuses disclosed in these publications, an image is formed in accordance with a predetermined portion of a recording sheet being sent, and the image is transferred onto the mentioned portion of the recording sheet. Therefore, stopping the recording sheet temporarily and regulating the time for feeding the recording sheet again becomes unnecessary. In short, these apparatuses are adapted to form an image in accordance with the time at which a recording sheet is sent out, and transfer the image onto an accurate portion on the surface of the recording sheet. Therefore, even when a tab and the like is provided on a front end of the recording sheet, inconveniences do not occur during the sending of the recording sheet.